1. Field of the Invention
This invention relates to trip mechanisms having two magnetic coils in a circuit breaker including positive temperature coefficient resistivity elements (PTC elements).
2. Description of the Related Art
A circuit breaker protects circuits not only in short circuit situations but also in overload situations. For instance, according to UL489 requirements, the circuit breaker must trip within an hour when current reaches 135% of the ampere rating of the breaker. Typically, a bimetal is used in existing circuit breakers for overload protection. The bimetal is a current carrying part in low ampere rated circuit breakers. When an overload situation occurs, the high current increases the temperature of the bimetal and the bimetal is deflected by the heat, causing the circuit breaker to trip.
Circuit breakers including bimetal must be calibrated which significantly raises the cost of manufacturing and include many other disadvantages related to using the bimetal and calibration. However, even with calibration the bimetal does not always behave consistently and a calibrated circuit breaker will not always trip at the set overload rating.
The method and apparatus of the present invention uses one or more conductive polymer elements such as a positive temperature coefficient resistivity element (PTC element) to replace the bimetal in a circuit breaker. The method and apparatus of the present invention also includes a smaller novel trip mechanism than those typically used in circuit breakers having a bimetal element, wherein the trip mechanism includes two magnetic coils on a single yoke.